On the basis of an invitation from the National Institute of Child Health and Human Development (NICHD), we are submitting an application to participate with the NICHD under a Cooperative Agreement in an ongoing multicenter clinical study designed to investigate problems in clinical obstetrics, particularly those related to prevention of low birth weight. The objective of this study is to facilitate resolution of these problems by establishing a network of centers that, by vigorous patient evaluation using common protocols, can study the required numbers of patients and can provide answers more rapidly than individual centers acting alone.